Re: [PATCH v3 1/2] tools/xenstore: set oom score for xenstore daemon on Linux

[Juergen Gross <jgross@xxxxxxxx>]

On 30.07.21 15:26, Ian Jackson wrote:
> Juergen Gross writes ("[PATCH v3 1/2] tools/xenstore: set oom score for xenstore 
> daemon on Linux"):
> > Xenstored is absolutely mandatory for a Xen host and it can't be
> > restarted, so being killed by OOM-killer in case of memory shortage is
> > to be avoided.
> >
> > Set /proc/$pid/oom_score_adj (if available) per default to -500 (this
> > translates to 50% of dom0 memory size) in order to allow xenstored to
> > use large amounts of memory without being killed.
> ...
> > +## Type: integer
> > +## Default: 50
> > +#
> > +# Percentage of dom0 memory size the xenstore daemon can use before the
> > +# OOM killer is allowed to kill it.
> > +#XENSTORED_OOM_MEM_THRESHOLD=50
> > +
> >   ## Type: string
> >   ## Default: @LIBEXEC@/boot/xenstore-stubdom.gz
> Thanks for working on this.  I approve of the principle.
>
> I have one question about detail:
>
> >         }
> > +       [ -z "$XENSTORED_OOM_MEM_THRESHOLD" ] || XENSTORED_OOM_MEM_THRESHOLD=50
> > +       XS_OOM_SCORE=-$(($XENSTORED_OOM_MEM_THRESHOLD * 10))
> > +
> > +       rm -f @XEN_RUN_DIR@/xenstored.pid
> ...
> > +       XS_PID=`cat @XEN_RUN_DIR@/xenstored.pid`
> > +       echo $XS_OOM_SCORE >/proc/$XS_PID/oom_score_adj
> The effect of all this is that the value specified in
> XENSTORED_OOM_MEM_THRESHOLD is transformed before being echoed into
> /proc, by being multiplied by -10.
Yes.

> Of course an alternative would be to ask the user to specify the
> tuneable directly but given its rather more obscure semantics I think
> it is reasonable to have this done by the script.
Correct. Otherwise the user would need to know about the oom_score_adj
ABI.

> But maybe we could add something to the doc comment ?
>
> Eg
>    # (The specified value is multiplied by -10 and echoed into
>    # /proc/PID/oom_score_adj.)
>
> ?
Why? This is an internal implementation detail. I don't see why the
user needs to know how this is accomplished. What is unclear with the
XENSTORED_OOM_MEM_THRESHOLD semantics as described?

There is no other parameter with an explanation how it's semantics are
being accomplished.


Juergen

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